Other congenital abnormalities arise from destruction of already developed structures. Hydranencephaly refers to destruction of the majority of the cerebral hemispheres secondary to a massive ischemic event thought to be bilateral internal carotid artery occlusion. Thin walled cystic structures replace the cerebral parenchyma. Posterior circulation structures are typically intact because they are supplied by the vertebrobasilar system. The presence of a falx cerebri and separation of the thalami differentiates hydranencephaly from severe alobar holoprosencephaly. It is sometimes however impossible to differentiate it from severe hydrocephalus.